Induction heating involves the heating of a nominally electrically-conducting material (i.e., a metal charge) by eddy currents induced by a time-varying electromagnetic field. Typically, the metal charge is placed in a furnace comprising a refractory crucible surrounded by a liquid-cooled copper coil. Electrical power in the form of alternating current from 50 Hz to 60,000 Hz is supplied to the coil from a suitable power supply. This creates an alternating electromagnetic field around the coil. Heat is produced in the metal charge by means of the eddy currents induced in the charge by the electromagnetic field surrounding the coil.
One type of power source for supplying the high frequency ac is a solid state power supply which utilizes high-power thyristor solid-state devices such as silicon-controlled rectifiers (SCRs). A block diagram of a typical induction heating apparatus (e.g., an induction furnace) and an inverter power supply for the furnace which employs SCRs is described and depicted in FIGS. 1 and 2 of U.S. Pat. No. 5,165,049. That patent is herein incorporated by reference in its entirety.
An induction furnace is one type of inductive load. U.S. Pat. No. 5,165,049 describes the problem of matching the resonant frequency of the inductive load with the resonant frequency of the inverter power supply so as to maximize energy transfer therebetween, in view of the constantly changing resonant frequency of the load and the need to avoid making the frequencies exactly identical. If the frequencies are made identical, the power supply will short out for reasons discussed in that patent. As that patent also describes, the resonant frequency of the load constantly varies. For example, the resonant frequency of a metal charge in a furnace varies as the composition changes and as it is heated, cooled, and as metal is added to or removed from the crucible. U.S. Pat. No. 5,165,049 also discloses circuitry for solving the problem of identical frequency of power supply and load by varying the phase difference between the current and voltage in the load in response to the resonant frequency of the load. The circuitry monitors zero-crossings of the current in the inverter, and generates a time delay before the inverter SCRs are fired in such a way that the output power level is maintained and at least a minimum phase shift .phi. is always maintained between current and voltage delivered to the load. In this manner, the SCR firing frequency (the inverter frequency) will always be different from the resonant frequency of the load.
As described in U.S. Pat. No. 5,165,049, the power transferred from the inverter to the furnace (assuming that the current is a sine wave and the voltage a square wave, as would be the case with an ideal inverter-type power supply) will be equal to: EQU P=(2/.pi.).times.(VI cos .phi.) (Equation 1)
where:
V=inverter voltage (=V.sub.DC for a full-bridge inverter)
I=amplitude of inverter current
.phi.=phase shift or phase angle between voltage and current
For .phi. between 0.degree. and 90.degree., an increase in .phi. will cause a decrease in power transferred to the furnace. Maximum power transfer will occur when .phi.=0 (i.e., when cos .phi.=1). However, as explained in U.S. Pat. No. 5,165,049, .phi. should not be allowed to reach zero so as to avoid shorting out the power supply. The control system in U.S. Pat. No. 5,165,049 maintains .phi. at a value safely distanced from zero (i.e., the minimum phase shift referred to above), but as close to zero as possible so that cos .phi. is as large as possible, thereby ensuring the maximum possible power transfer.
When the resonant frequency of the load varies, such as when there is a change in the composition, temperature or amount of metal charge, the control circuitry in U.S. Pat. No. 5,165,049 readjusts the phase angle .phi. to maintain it at its optimum value for maximum power transfer.
FIGS. 1(a) and FIGS. 1(b) herein depict a simplified example for illustration purposes only of load current waveforms at different periods of time.
FIG. 1(a) shows time period T.sub.1 wherein the resonant frequency of the load is 1 Hz. The phase angle (i.e., the time delay between a zero crossing at t.sub.0 and the firing pulse at t.sub.1) is .phi..sub.1. In this example, .phi..sub.1 is approximately 36.degree. (the full cycle representing 360.degree.), yielding a power transfer value of approximately 0.81 (cos .phi.=cos 36.degree.=0.81).
FIG. 1(b) shows time period T.sub.2 wherein the resonant frequency of the load has changed to 2.0 Hz due to some change in the metal charge (e.g., composition, temperature or amount). In reality, a 100% change in resonant frequency is unlikely to occur over short periods of time with respect to the same furnace. Nor would such a large change ever occur with respect to certain furnaces under even drastically changing conditions. However, such a large change is hypothesized to better illustrate an important point about the effects of a change in the phase angle. In FIG. 1(b), the phase angle .phi..sub.2 has now doubled to approximately 72.degree., yielding a power transfer value of approximately 0.31 (cos .phi.=cos 72.degree.=0.31). Thus, the change in resonant frequency will cause a reduction in the power transfer unless the phase angle is readjusted. In this instance, the phase angle should be reduced so that optimum power transfer efficiency is maintained. In a case where changes in the load causes the resonant frequency to decrease, the phase angle should be increased to avoid having it approach too close to zero.
The control circuitry in U.S. Pat. No. 5,165,049 functions to increase or decrease the phase angle as the resonant frequency of the load varies, thereby maintaining optimum power transfer efficiency. This control circuitry typically requires a few waveform cycles to complete the adjustment in phase angle because of the time delay inherent in its feedback mode of operation. For example, in a more realistic example, the resonant frequency of a furnace might vary from 1000 Hz to 1030 Hz due to addition of some metal charge. An adjustment delay of three or four cycles would only take 3-4 milliseconds. Also, since the phase angle need only be changed by a very slight amount, no significant problems are encountered in generally maintaining a relatively steady and optimum transfer of power at all times. Furthermore, any changes in the resonant frequency will likely occur gradually. For example, resonant frequency changes caused by adding or removing metal charge or by varying the temperature of the charge are likely to occur over the course of seconds or minutes, compared with the adjustment delay time or lag time of the phase angle adjusting circuitry. In sum, a single inverter/single load configuration will rarely ever encounter power transfer problems associated with changes in the resonant frequency of the load.
In U.S. Pat. No. 5,165,049, a single induction load (e.g., a single furnace) is supplied by a single inverter power supply. It is sometimes desired to connect a single inverter power supply to plural induction loads, for example, to two or more furnaces or zone heaters, thereby avoiding the expense that would be associated with powering each induction load with a separate power supply. However, a problem arises when a single inverter power supply is switched among plural induction loads. Since each induction load will most likely have significantly different resonant frequencies at any given point in time, the phase angle referred to above will have to be constantly adjusted by relatively large amounts (as compared to very small adjustment amounts when using a single induction load) each time the power supply is switched to a new load. Furthermore, the phase angle adjustment should occur almost instantaneously if the power supply is distributing the power by switching from one load to the next in a time multiplexed manner. For example, instead of FIGS. 1(a) and FIGS. 1(b) representing the varying resonant frequency of a single load at different periods of time, these figures might represent two different loads having significantly different resonant frequencies which are sequentially connected to a single inverter power supply in a time multiplexed manner.
If the phase angle adjusting circuitry in U.S. Pat. No. 5,165,049 were employed for switching a single inverter power supply between two different loads having significantly different resonant frequencies, the inherent time delay associated with its feedback manner of operation would result in a significant temporary overshoot or undershoot of power delivered to the load during every switching. Even though the phase angle will eventually be adjusted by the circuitry to optimize the power transfer for the new load, the time period for adjustment will span several waveform cycles and will be long enough to cause a significant temporary overshoot or undershoot of power. Although this overshoot and undershoot also occurs when a single load undergoes changes in its resonant frequency, the amount of overshoot and undershoot will typically be insignificant (and often so small as to be undetectable) because any phase angle changes caused by changes in the parameters of the metal charge will usually be very small and will occur over relatively long periods of time as compared to the lag time of the phase angle adjusting circuitry.
Large overshoots have the potential of damaging critical circuit components. Large overshoots and undershoots also distort the desired distribution of power among plural loads by introducing uncontrollable and unpredictable changes or fluctuations of power transfer efficiencies. Accordingly, there is a need in the art to reduce the amount of overshoot and undershoot caused by switching among plural loads sharing a single inverter power supply. There is also a need to provide such a function in conjunction with existing phase angle adjusting circuitry in U.S. Pat. No. 5,165,049. The present invention fills these needs.